Retractable assemblies are widely used in analytical measurement technology. They serve for removing sensors, without process interruption, from the process, even at process pressures up to 10 bar and above, and for subsequently inserting sensors back into the process. In accomplishing these goals, the sensor is moved manually or automatically between the measuring position and the service position. In the service position, the sensor can be checked, calibrated, cleaned or replaced.
Especially in the case of automatically working retractable assemblies, in which case the sensor is moved by means of pressure-bearing air using a pneumatic linear drive, such being known, for example, from DE 10 2006 010 810 A1, there is the danger that, in the case of service, a sensor will be removed and then, subsequently, it will be forgotten to install a replacement sensor. If, then, the linear drive is activated, in order to shift the assembly into the measuring position, it is possible for process medium to escape through the now-open, tubular holder, in which the sensor is missing. The process medium can have a pressure of 10 bar and a temperature up to 140° C. Furthermore, the process medium can be chemically aggressive. A defective servicing of this type can lead to not insignificant accidents. Known from DE 102 41 833 A1 is a retractable assembly, in the case of which an axial movement of the immersion tube from the service position is prevented by means of a safety device, when the sensor is missing. For this, the retractable assembly must be equipped with additional structural features.